<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .box1 {
            width: 100px;
            height: 100px;
            margin: 100px auto;
            position: relative;
            font-size: 16px
        }

        #box2 {
            position: absolute;
            width: 50px;
            height: 50px;
            top: 20px;
            left: 15px;
            font-size: 13px;
            background: goldenrod;
            display: none;
        }
    </style>
</head>

<body>
    <div class="box1">
        <input type="checkbox" id="btn">记住密码
        <div id="box2">要谨慎的啊</div>
    </div>
</body>
<script>
    (function () {
        var btn = document.getElementById('btn');
        var box2 = document.getElementById('box2');
        btn.onmouseover = function () {
            box2.style.display = 'block';
            setTimeout(function () {
                box2.style.display = 'none';
            }, 5000)
        }
    })();
</script>

</html>